1. Field of the Invention
This invention relates to a process for producing an electroluminescent device. More particularly, it relates to a process for producing an electroluminescent device which is used for segment display or matrix display of a light-emitting type in measuring instruments, or a display of various information terminal devices.
2. Description of the Related Art
Electroluminescent devices (hereinafter referred to as "EL devices") according to the prior art utilize the phenomenon that light is emitted when an electric field is applied to a phosphor such as zinc sulfide (ZnS), etc., and have drawn increasing attention as a component for constituting a light-emitting flat-panel display.
FIG. 7 of the accompanying drawings is a schematic view showing a typical sectional structure of an EL device 10 according to the prior art.
This EL device 10 is formed by sequentially laminating a first electrode 2 consisting of an optically transparent ITO film, a first insulating layer 3 made of tantalum pentaoxide (Ta.sub.2 O.sub.5), etc., a luminescent layer 4, a second insulating layer 5 and a second electrode 6 consisting of the ITO film on a glass substrate 1 as an insulating substrate.
The ITO (Indium Tin Oxide) film is a transparent conductor film obtained by doping tin (Sn) into a indium oxide (In.sub.2 O.sub.3) and having a low resistivity. Accordingly, this film has seen wide application in the past as a transparent electrode.
A material used for the luminescent layer 4 contains strontium sulfide as a host material, and cerium (Ce) added as a luminescent center.
Light emission colors of the EL device are determined by the kind of dopants in strontium sulfide, and when cerium (Ce) is added as the luminescent center, for example, light emission of a blue-green color can be obtained. When this blue-green color light emission is passed through a filter which cuts light having a wavelength of longer than 500 nm, for example, blue light emission can be obtained.
To obtain blue color emission light in the EL device 10 having the structure described above, a sulfur compound of an alkaline earth metal such as strontium sulfide containing cerium (Ce) added thereto or strontium selenide and a selenium compound have been used in the past as the constituent material of the luminescent layer 4. As the method of producing the luminescent layer 4, electron beam (EB) vacuum deposition and sputtering have been examined, and recently, CVD (Chemical Vapor Deposition) has also been examined.
In this CVD process, a method of transporting a heated metal by a halogen element gas and a method of vaporizing a halogen compound at a high temperature and transporting it into a reaction furnace are known as the method of supplying the alkaline earth metal.
According to the methods described above, however, the vaporization temperature is high and the control of the vaporization quantity is difficult. Further, there is a high possibility that the ratio of the gases of the Groups II and VI to the luminescent center substance supplied into the reaction furnace may change, so a high quality light emission layer cannot be obtained stably.
As a method of solving this problem, the method described, e.g., in SID 91 DIGEST, pp. 282-285, which uses chemical vapor deposition using Sr(thd).sub.2 as a precursor for strontium can be used. Generally, Sr(thd).sub.2 has a melting point of not higher than 250.degree. C., and can improve temperature controllability to obtain a vapor of the precursor in comparison with the halogen compounds described above.
However, thd (2,2,6,6-tetramethyl-3,5-heptanedione) chelates of alkaline earth metals have a characteristic property such that when the temperature for heating the precursors is increased, the vaporization quantity also increases, but they are decomposed before a sufficient vaporization quantity is obtained.
Accordingly, in order to transport the precursor into the reaction furnace without causing the change of the form of the precursor, heating and vaporization must be attained at a temperature below the decomposition temperature and in this case, a sufficient feed quantity of the precursor for the alkaline earth metal cannot be obtained.
When a thin film of strontium sulfide (SrS) or strontium selenide (SrSe) is formed, for example, a sufficient film growth rate cannot be obtained, and the problem that the film formation time is too long occurs.
Further, when dopants such as the luminescent center are added into the thin film, their amounts must be controlled so that they become about a few at% on the basis of the atomic weight of the thin film. When the composition of the dopants such as the luminescent center changes even slightly, the light emission characteristics of the resulting EL device greatly change, and reliability is greatly deteriorated. In order to add a predetermined amount of the dopants to the thin film in a uniformly distributed state, therefore, the feed quantity must be controlled to a lower level than the host material constituent gas, and a large-scale apparatus becomes necessary for delicate control of the amount of the vaporization temperature of the precursors for feeding the dopants.
Because of various problems described above, it has not been possible to obtain a sufficient feed quantity of the precursor for the alkaline earth metal compounds, and to improve the thin film growth rate. In other words, a luminescent layer thin film having high reliability cannot be obtained by chemical vapor deposition.